Unfortunately, reports that circulated two years ago were never getting confirmation from Apple. Apple previously used a remote data storage system offered by Amazon Web Services and Microsoft Azure.
Meanwhile, the March 2017 edition of Apple iOS Security Guide is still listed on the Microsoft Azure service and not on the Google Cloud Platform.
The new edition describes its iCloud service as a number of stored encrypted files, without user identification information, using third-party storage services, such as (Amazon) S3 and Google Cloud Platform.
The report does not specify details of Apple's decision to use Google's cloud service other than to share photos and videos, or related to the start date for the transfer of storage services for the iCloud.